Lime Softening Sources, Have Relatively Stable Water Which and the Complexity of the Chemistry of Variable Source Water Quality Quality

A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET Lime Softening Summary Hard water can cause scaling problems in water heaters and soap does not lather well in hard water. Therefore, some water utilities soften water to improve its quality for domestic use. Lime softening is best suited to groundwater sources, which have relatively stable water quality. The combination of variable source water quality and the complexity of the chemistry of lime softening may make lime softening too complex for small systems that use surface water sources. Although lime softening has been used successfully by groundwater systems serving fewer than 3,000 people, it is unlikely to be suitable for ○○○○○○○○○○○○ treating groundwater in systems serving 500 or fewer people unless those systems have some form of contract or satellite operation that would enable a trained operator to monitor the treatment process. Prefabricated lime softening equipment is available for small systems. Also, there is an American Water Works Association Standard for quicklime and hydrated lime (ANSI/AWWA B202-93) that provides purchasers, manufacturers, and suppliers with the minimum requirements, including physical, chemical, packaging, shipping, and testing requirements. Either hydrated lime [Ca(OH)2] or quicklime (CaO) may be used in the softening process. The choice depends upon economic factors, such as the○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ relative cost per ton of the two materials as well as the size and equipment of the softening plant. Hydrated lime is generally used more in smaller plants because it stores better and does not require slaking (producing a chemical change in lime by combining it with water) equipment. On the other hand, quicklime costs less per ton of available calcium oxide and is thus more economical for use in large plants. Softened water has high causticity and scale-formation potential; hence, recarbonation is employed to reduce pH and mitigate scaling of downstream processes and pipelines. Onsite combustion generation of carbon dioxide (CO2) or liquid CO2 is the most common source of carbon dioxide for recarbonation. What is hard water? noncarbonate hardness. Carbonate hardness is “Hardness” in water is primarily the result of the portion of total hardness present in the concentrations of calcium and magnesium. form of bicarbonate salts [Ca(HCO3)2 and Thus, some water utilities remove calcium and Mg(HCO3)2] and carbonate compounds (CaCO3 magnesium to soften the water and improve its and MgCO3). quality for domestic use. Other ions that pro- Noncarbonate hardness is the portion of cal- duce hardness include iron, manganese, cium and magnesium present as noncarbonate strontium, barium, zinc, and aluminum, but salts, such as calcium sulfate (CaSO ), calcium these ions are generally not present in signifi- 4 chloride (CaCl ), magnesium sulfate (MgSO ), OF FOUR PAGE cant quantities. Therefore, total hardness is 2 4 and magnesium chloride (MgCl). The sum of usually defined as the sum of magnesium and carbonate and noncarbonate hardness equals calcium hardness in milligrams per liter (mg/L), one total hardness. as calcium carbonate (CaCO3). Total hardness can also be differentiated into carbonate and NATIONAL DRINKING WATER CLEARINGHOUSE Lime Softening ○○○○○○○○ What may be “hard” water in one area may used and may reduce the carbon dioxide level to be perfectly acceptable water in another area. 10 mg/L or less. Aeration also oxidizes iron and In general, the degree of hardness is classified manganese; clogging of the aeration tray is a as follows: problem. ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Hardness mg/L as CaCO Presedimentation is used primarily by those 3 plants treating high turbidity surface waters. Soft 0 to 75 Moderate 75 to 150 What are the monitoring and Hard 150 to 300 operating requirements? Very hard Above 300 Regulatory monitoring requirements for lime softening plants depend on whether the source Source: Water Treatment Plant Design, 1998. water is surface water or groundwater. Process For most applications, total hardness of 120 monitoring requirements should focus on mg/L or less and magnesium hardness of 40 measurement of pH, hardness, and alkalinity mg/L or less appear to be acceptable design for plants treating groundwater. In addition, criteria for softening facilities. filtered water turbidity monitoring is needed at the plants treating surface water for compliance purposes, as well as to manage filter operation. How does the lime-softening process work? One of the difficult aspects of lime softening is In the lime-softening process, the pH of the the operation and maintenance of lime feeders water being treated is raised sufficiently to and lines carrying lime slurry to the point of precipitate calcium carbonate and, if necessary, application. In addition, plant operators must magnesium hydroxide. The normal pH of water understand lime softening chemistry. Measure- is between 6.5–8.5. In small systems, lime ment of pH must be accurate, and the operator softening is typically practiced by adding must know that the pH meter is properly hydrated lime to raw water to raise the pH to calibrated. Failure to maintain the proper pH in approximately 10. This removes calcium car- softened water prior to filtration at a lime bonate, essentially limestone. If magnesium softening plant could result in precipitation or removal is also required, the pH during soften- excess lime in the filter beds and formation of ing needs to be closer to 11. Lime-softening calcium carbonate deposits within the filters. systems need not be pilot tested for small systems using groundwater sources. Jar testing What are the chemical to determine appropriate process pH and requirements? chemical doses is sufficient. Doses of these The amount of lime required to remove carbon- chemicals should not change greatly over time ate hardness and magnesium can be calculated unless the groundwater is subject to periodic using in the following equation: infiltration by surface water that changes in quality. Lime-softening systems do need to be CaO (lb/mil gal) = 10.6 CO2 (mg/L) + 4.7 [alkalinity pilot tested if used on surface water sources (mg/L) + magnesium hardness (mg/L) + X] with variable quality. Where CaO is 100 percent pure, CO2 is expressed as CO2, alkalinity is expressed as What pretreatment is used? CaCO3, and X is the required excess hydroxide The principal types of pretreatment used before alkalinity in mg/L as CaCO3. The magnesium lime softening are aeration and presedimentation. hardness shown is the amount to be removed by softening, and not the amount present. Aeration may be used to remove carbon dioxide Desired excess alkalinity can be determined from the source water before softening. This is from the magnesium hydroxide solubility only applicable to groundwaters where carbon relationship: it is typically in the range of 30 to dioxide concentrations are relatively high. Lime 70 mg/l and is often estimated at 50 mg/L removal of carbon dioxide in source water adds expressed as CaCO . to operation costs because of chemical expenses 3 two and increased calcium carbonate residuals. PAGE OF FOUR PAGE Induced draft or open tray aeration is often EIGHT • June 1998 What are other softening ○○○○○○○○ • removing radium 226 and 228, alternatives? • removing arsenic and uranium, The selection of lime, lime-soda ash, or caustic • removing heavy metals, soda softening is based on cost, total dissolved • supplementing disinfection and reducing solids criteria, sludge production, carbonate algal growths in basins, and noncarbonate hardness, and chemical ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ • removing certain organic compounds and stability. Water containing little or no reducing total organic carbon (TOC), noncarbonate hardness can be softened with • removing silica and fluoride, lime alone. However, water with high • removing iron and manganese, noncarbonate hardness may require both lime • reducing turbidity of surface waters in and soda ash to achieve the desired finished conjunction with the hardness precipitation water hardness. Softening with lime or lime- process, soda ash is generally less expensive than • increasing the Langelier Saturation Index, caustic softening. Caustic soda softening useful for corrosion control under some increases the total dissolved solids of treated conditions, and water, while lime and lime-soda ash softening • possibly removes Giardia Lamblia cysts. often decrease total dissolved solids. Caustic soda softening produces less sludge than lime Are there concerns about soft and lime-soda ash softening. Caustic soda does water? not deteriorate during storage, while hydrated In most cases, for consumer use, relatively soft lime may absorb carbon dioxide and water water is preferable; however, it does have during storage, and quicklime may slake in several disadvantages. Excessively soft water storage causing feeding problems. The final can cause corrosion in pipes. This corrosion can selection is generally based on cost, water shorten the service life of pipes and household quality, and owner and operator preference. appliances and can result in toxic materials, such as lead and cadmium, being dissolved in drinking water. The decision to soften a water What are the benefits of supply depends completely on the community. softening? Drinking water regulations do not generally Potential benefits of softening water at a central require softening. treatment plant include

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